Inorganic Carbon-14
Matt Baillie
3/25/04
HWR696T
Outline
Production of 14C Variance through time of 14C production How to get 14C into groundwater Complications and corrections Conclusions
Production in the atmosphere
14C produced through secondary spallation reactions between neutrons and 14N atoms
14C atoms then quickly combine with O2 to form 14CO2
Subsurface production unimportant due to CO2 in soil
From (Taylor, 2000)
Temporal production variance
Variation in production of 14C in the atmosphere dependent on cosmic ray flux, which is in turn dependent on solar activity, geomagnetic field, etc.
Atmospheric production can be calibrated using dendrochronology, as well as U-Th dating of corals
Industrial age burning of fossil fuels has put a huge amount of “dead” carbon into the atmosphere, diluting atmospheric 14C
Atmospheric testing of nuclear weapons increased (up to double) the 14C in the atmosphere Now approaching previous levels due to moratorium on
atmospheric testing, as well as 14CO2 going mostly into the oceans
Temporal production variance
Temporal production variance
Getting 14C into groundwater
14CO2 incorporated into plants through photosynthesis, undergoing depletion
14C is passed from plants to soil, and becomes slightly enriched due to the diffusion of 12CO2 into the atmosphere
Soil CO2 levels are 10-100 times greater than atmospheric CO2 levels, so absolute amounts of 14C are much higher in the soil than in the atmosphere
Getting 14C into groundwater
In open system conditions (contact with the soil), 14C is replenished, and remains slightly enriched from soil levels
In closed system conditions, 14C is no longer replenished by the soil, and begins to decay away
Getting 14C into groundwater
Getting 14C into groundwater
Once the 14C is in closed system conditions and assuming no other processes affect it subsequently, the groundwater can be dated using the equation:
where t is the mean residence time of the groundwater, at is the activity of the 14C at the time of sampling, and a0 is the initial activity of 14C
Ca
Cat14
0
14
ln8267t
Complications
What was the initial 14C activity in the atmosphere when the groundwater entered closed system conditions?
Carbonate dissolution introduces “dead” carbon into the groundwater, taking 14C-active carbon out of the groundwater
Matrix diffusion of 14C into dead-end pores decreases 14C in groundwater
Reduction of organics by sulphate adds 14C-free carbon to the groundwater
Geogenic (mantle/deep crust) 14C-free CO2
Methanogenesis introduces “dead” carbon
Corrections
To correct the calculated 14C age, apply a correction factor, q:
Caq
Cat14
0
14
ln8267t
Corrections
Initial activity can be determined through the variations in atmospheric 14C through time
Corrections
Matrix diffusion: correction based on matrix porosity and fissure porosity in a dual-porosity aquifer
Sulphate reduction: stoichiometric correction
Geogenic CO2: δ13C correction Methanogenesis: δ13C and
stoichiometric correction
f
p
apparentreal
nn1
tt
SH(1,2)DICDIC
q2
SH2 mm
m
geo13
rech13
geo13
measDIC13
geo CδCδ
CδCδq
meas
4measCH DIC
CH2DICq
4 m
mm
Corrections
For carbonate dissolution, correction factors are more complicated, and there are therefore several different correction models that can be applied Statistical correction Alkalinity correction Chemical mass-balance correction δ13C mixing (δ13C model) Fontes-Garnier model
Carbonate corrections
Statistical correction Simple geometric correction based on the type of aquifer
system: 0.65-0.75 for karst systems 0.75-0.90 for sediments with fine-grained carbonate such as loess 0.90-1.00 for crystalline rocks
(from Vogel, 1970)
Can be estimated by: for any given recharge area
Limited in usefulness to waters found near the recharge area
soil14
DIC14
STAT Ca
Caq
Carbonate corrections
Alkalinity correction Correction based on the initial and final DIC concentrations (from
Tamers, 1975)
Assumes fully closed system conditions, with no exchange between the groundwater and the soil CO2 during dissolution
Model is of “limited interest” (Clark and Fritz, 1997)
332
332ALK HCOCOH
HCO21COHq
mm
mm
Carbonate corrections
Chemical mass-balance correction Closed-system model, with dissolution below the water table and
no exchange with soil CO2
Estimated by:
With mDICrech being estimable from the pH of the recharge area, and:
mDICfinal = mDICrech+[mCa2++mMg2+-mSO42-+1/2(mNa++mK+-mCl-)]
Only useful in geochemically simple systems with no carbonate loss from the groundwater
final
rech
DIC
DICq
m
m
Carbonate corrections
δ13C mixing (δ13C model) Uses 13C as a tracer, useful in open and closed systems. First introduced by Pearson (1965) and Pearson and Hanshaw
(1970), later modified to work at higher pH (7.5-10):
Enrichment factor chosen for the soil greatly affects groundwater age, and is based on pH in the recharge area; assumes that this pH was the same when the groundwater was originally recharged
carb13
rech13
carb13
DIC13
Cδ CδCδ
CδCδq 13
Carbonate corrections
Fontes-Garnier model (1979; 1981) Calculates q based on both chemistry and δ13C values of
groundwater Uses Ca and Mg concentrations as a proxy for carbonate
dissolution, as well as δ13C to partition the carbon into DIC that has exchanged with soil CO2 and that which has not
Does not take into account DIC sources aside from carbonate dissolution and soil CO2 exchange
meas
exchCOcarbmeasGF DIC
DICDICDICq 2
m
mmm
Conclusions
Inorganic 14C is a useful tool for determining mean residence time of groundwater IF: Initial 14C activity is known Recharge conditions can be determined Conditions within the aquifer are somewhat known (in relation to
carbonate dissolution) Groundwater is not too old for the method to be useful (for all
practical purposes, water must be at most 30,000 years in residence (Clark and Fritz, 1997))
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